Diagnostic Use
When osmolality is requested on a sample, this will be measured by an osmometer.
Calculated osmolality
When sodium, urea and glucose results are available, these can be used to calculate an estimated osmolality.
This can be a useful pointer to the presence of ethanol, mannitol or other solutes in the plasma.
The following formula is used in the laboratory computer calculation.
Calculated omolar gap = (Na x 2) + Urea + Glucose (Smithline-Gardner formula)
Normal range: 275 – 290
- Osmolar Gap : The osmolar gap is the difference between the measured and calculated serum osmolalities. The normal range for the osmolar gap is 0 – 15. An increased osmolar gap may be due to the presence of mannitol, ethanol, methanol, ethylene glycol, or other substance.
- The calculated osmolarity will differ from the measured osmolality in specimens with high lipid (lipaemic) or protein (e.g. myeloma) concentrations. This is due to pseudohyponatraemia, an apparent low measured sodium concentration (if measured by main automated analyser, using the indirect method). Measured osmolality is not affected by an increased lipid or protein.
Test Method
Principle: Freezing point depression
Analyser: Nova Osmo1
The calculated osmolality formula was updated in June 2026 on LIS from Bhagat formula 1.89(Na) +1.38(K) + 1.08(Glu) + 1.03(Urea) + 7.45 to Smithline-Gardner formula 2(Na) + Glu + Urea (1)
Uncertainty of Measurement
1.5% at 300 mmol/kg of water
1% at 812 mmol/kg of water